1. Field of the invention
The present invention relates to biotechnology, and specifically to a method for producing an L-amino acid, such as L-histidine, by fermentation. The present invention specifically relates to a gene derived from an Escherichia coli bacterium. The gene is useful for improving production of L-histidine.
2. Description of the Related Art
Conventionally, L-amino acids have been industrially produced by fermentation utilizing strains of microorganisms obtained from natural sources or mutants of the same modified to enhance L-amino acid productivity.
Many techniques have been reported regarding enhancement of L-amino acid production, for example, by transformation of a microorganism by recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). These techniques are based on increasing the activities of the enzymes involved in amino acid biosynthesis and/or desensitizing target enzymes from feedback inhibition by the produced L-amino acid (see, for example, Japanese Laid-open application No. 56-18596 (1981), WO 95/16042 or U.S. Pat. Nos. 5,661,012 and 6,040,160).
In the L-histidine biosynthetic pathway, imidazoleglycerol phosphate synthase encoded by the hisH and hisF genes catalyzes reaction whereby an intermediate compound, 5′-phosphoribosyl-4-carboxamide-5-aminoimidazole (AICAR), is released. At the same time, the initial reaction in the L-histidine biosynthetic pathway is catalyzed by the HisG protein and involves a displacement on C-1 of phosphoribosylpyrophosphate (PRPP) by N-1 of the purine ring of adenosine triphosphate (ATP). However, AICAR is not only released upon histidine biosynthesis but it is also the precursor in the biosynthesis of purines and, consequently, purine nucleosides and nucleotides, such as AMP and ATP. So, recycling of AICAR to AMP is an important process in L-histidine production.
The purH gene encodes a bifunctional enzyme having AICAR transformylase activity ((also known as 5′-phosphoribosyl-4-carboxamide-5-aminoimidazole transformylase) [EC 2.1.2.3] and IMP cyclohydrolase activity [EC 3.5.4.10]. The latter activity catalyzes the penultimate and final steps in the de novo synthesis of inosine monophosphate (IMP) (Escherichia coli and Salmonella, Second Edition, Editor in Chief: F. C. Neidhardt, ASM Press, Washington D.C., 1996).
There have been no reports to date, however, describing enhancement of the dual activities of AICAR transformylase-IMP cyclohydrolase for the purpose of improving L-histidine production using strains of the Enterobacteriaceae family.